SCIENCE AND PRACTICE. 353 



do occur in the single coatings are small, and for the most 

 part well surrounded by gutta-percha, which is able to resist 

 the internal pressure of the air contained in them, under the 

 partial vacuum, and, being in an elastic medium, the external 

 pressure is only able to contract them for the moment. 



In all the knot lengths some hundreds in number 

 which we have tested under pressure, not a single fault has 

 been developed by this method. This will by no means 

 speak against the system, but in favour of the core tested. 

 Mr. Reid's argument, that if, in ten thousand lengths tested, 

 only one be found faulty by this method, which resists de- 

 tection by the ordinary process, the discovery will save infi- 

 nitely more than the cost of testing the whole, is perfectly 

 just ; but then the probability may be less than one in ten 

 thousand. 



There are two conditions, therefore, under which a cable 

 is generally operated upon electrically : first, under atmo- 

 spheric pressure ; and, secondly, under an augmented pres- 

 sure : both at an uniform temperature. 



There are two ways in which the core may become faulty : 

 the first is by injury sustained by the gutta-percha, or by the 

 thin places which would occur on one side if the wire, by 

 any mischance, became eccentric in the gutta-percha ; the 

 other kind of fault is of very rare occurrence in cables which 

 are not submerged, but has happened sometimes in the pro- 

 cess of sheathing : it is the rupture of the copper conductor 

 where it is joined, or where it is too brittle to accommodate 

 itself to some strain or sudden small elongation which the 

 cable can otherwise sustain. 



75. Electrical Measurement of the Copper Conductor of a 

 Cable. This may be done in two ways : 



(1.) By means of a differential galvanometer. 



(2.) By means of a Wheatstone's bridge. 



The first system is the simpler of the two. Differential 

 galvanometers are usually made with two coils of insulated 

 wire of equal and opposite magnetical effects upon the needle 

 suspended in their centre ; so that, on joining up the coils, 

 parallel to each other, and inserting them in the circuit of a 



A A 



